1. Field of the Invention
The present invention relates generally to an ignition system of capacitor discharge type for an internal combustion engine and more particularly to an ignition system for an internal combustion engine which is equipped with an electric generator adapted to start the generation of electricity in response to actuation, for example, of a kick starter of a two-wheeled automobile.
2. Description of the Related Art
Heretofore, such a capacitive discharge type ignition (CDI) system for an internal combustion engine (hereinafter also referred to simply as the engine) is well known in which a capacitor is previously charged with a boosted voltage for generating a spark discharge at an ignition plug through an ignition coil having a primary winding adapted to be energized by an electric current discharged from the capacitor. The capacitor discharge type ignition system features a property that the fuel mixture within the engine cylinder can be fired without fail even when the spark plug is in the contaminated state or even when the onboard battery is of a small capacity, because of short duration of the electric discharge and steep rise-up thereof.
In the capacitor discharge type ignition system mentioned above, there is employed as a booster circuit for charging the capacitor a DC-DC converter which is comprised of a booster transformer and a power transistor for turning on and off repeatedly the booster transformer
FIG. 4 is a circuit diagram showing a configuration of a hitherto known CDI type ignition system for an internal combustion engine of a two-wheeled automobile.
Referring to the figure, the ignition system includes a generator or dynamo 1 which is adapted to be driven by a kick starter upon starting of the engine and by the engine (not shown) after the starting thereof, a rectifier circuit 2 connected to the dynamo 1 for rectifying an AC power generated by the dynamo 1 to a DC power, a battery 3 connected to the output of the rectifier circuit 2 for supplying an electric power to the whole system, a DC--DC converter 4 connected to the output of the rectifier circuit 2 and the battery 3 for boosting up the DC voltage, an ignition capacitor 5 having one terminal connected to the output of the DC--DC converter 4 and charged with a boosted voltage supplied therefrom, an ignition coil 7 connected to the other terminal of the ignition capacitor 5 for discharging through the ignition coil 7 the electric charge stored in the capacitor 5 to thereby cause a spark plug to produce a spark, a thyristor 8 connected in series to the ignition coil 7 between the one electrode of the ignition capacitor 5 and the ground for turning on and off a discharge path extending to the ground through the ignition capacitor 5 and the ignition coil 7, and an ignition timing control circuit 10 connected directly to the gate electrode of the thyristor 8 and connected to the output of the rectifier circuit 2 and the buttery 3 via a power supply stabilizer circuit 9 for generating a trigger signal which is applied to the gate of the thyristor 8 for causing it to turn off or open the aforementioned discharge path for thereby causing the electric discharge to take place in the spark plug 6.
In the CDI type engine ignition system mentioned above, there is provided between the input and the output of the DC--DC converter 4 an oscillation stop transistor 11 for stopping oscillation of the DC--DC convert 4. Further, an electromagnetic pickup 12 is connected to the ignition timing control circuit 10, which pickup 12 is disposed in opposition to a rotating shaft of the engine (not shown) for generating an ignition control signal at every predetermined crank angle. Further, a fly-wheel diode 13 is connected in parallel to the ignition coil 7 for limiting a reverse current flow therethrough. Parenthetically, the ignition timing control circuit 10 may be implemented on the basis of a microcomputer, as is known in the art.
The DC--DC converter 4 is comprised of a transformer 41, an oscillation transistor 42 constituted by a power transistor, a resistor 43, a diode 44 and a feedback circuit 45 for sustaining the oscillation, interconnection of which will be apparent from FIG. 4.
The transformer 41 includes a primary winding 41a having one end connected to the battery 3 (and hence the output of the rectifier circuit 2) and the other end connected to a collector of the oscillation transistor 42, a charging secondary winding 41b wound with polarity opposite to that of the primary winding 41a and having one end from which a boosted voltage is outputted through the diode 44, and a switching secondary winding 41c wound with the same polarity as the primary winding 41a for turning on/off the oscillation transistor 42 at a high frequency on the order of 20 kHz through the feedback circuit 45. On the other hand, the oscillation transistor 42 has a base connected to the battery 3 via a resistor 43 and the output terminal of the switching secondary coil 41c via the feedback circuit 45 which is constituted by a resistor 45a and a capacitor 45b connected in series to each other.
The ignition coil 7 includes a primary winding 7a having one end connected to the ignition capacitor 5 and the other end connected to the ground potential and a secondary coil 7b having an output end connected to the spark plug 6.
The power supply stabilizer circuit 9 includes a transistor 93 which has a collector connected to the output of the battery 3, an emitter connected to the ignition timing control circuit 10 and a base which is connected to the battery 3 via a resistor 91 and to the ground via a constant-voltage diode 92 such as a Zener diode. Connected between the output terminal of the DC--DC converter 4 and the ground is a serial connection of resistors 14 and 15. The oscillation stop transistor 11 has a base connected to a junction between the resistors 14 and 15 and a collector connected to the base of the oscillation transistor 42, and an emitter connected to the ground. Further, the base of the transistor 11 is also connected to the output of the ignition timing control circuit 10 via a resistor 16.
The engine ignition system of the structure described can operate even when the battery voltage becomes low or unavailable due to power consumption.
Now, description will turn to operation of the CDI type ignition system described above.
Upon starting of the engine (not shown), the kick starter (also not shown) is actuated to revolve the dynamo 1, whereby an AC power is generated. The AC power thus generated is rectified by the rectifier circuit 2, whereby a DC voltage is generated and inputted to the DC--DC converter 4. The voltage inputted to the DC--DC converter 4 is applied to the primary winding 41a and the base of the oscillation transistor 42 via the resistor 43, which results in turn-on (conduction) of the oscillation transistor 42, allowing a current to flow through the primary winding 41a to the ground. Thus, voltages increasing gently are induced in the charging secondary winding 41b and the switching secondary winding 41c, respectively.
The voltage induced in the switching secondary winding 41c is fed to the base of the oscillation transistor 42 via the feedback circuit 45, which results in that the current flowing through the primary winding 41a is further increased. When the current flowing through the primary winding 41a has attained a saturation level, the boost-up operation stops with the base current of the oscillation transistor 42 decreasing due to the presence of the feedback circuit 45. As a consequence, the oscillation transistor 42 operates to decrease the current flowing through the primary winding 41a. In response to the decrease in the current flowing through the primary winding, the voltage induced in the switching secondary winding 41 assumes the reversed polarity, whereupon the oscillation transistor 42 is turned off immediately. As a consequence, the current flowing through the primary winding 41a decreases rapidly to zero, bringing about change of the magnetic flux in the opposite direction, which in turn causes a high voltage of reversed polarity to be induced in the charging secondary winding 41b. With this voltage, the ignition capacitor 5 is electrically charged via the diode 44. Subsequently, the secondary current decreases to zero, whereupon the base voltage of the oscillation transistor 42 is restored by the output from the rectifier circuit 2.
As the operation described above is repeated, the ignition capacitor 5 is charged progressively, being accompanied with increase in the terminal voltage thereof. When a voltage derived by dividing the capacitor terminal voltage by the division circuit composed of the resistors 14 and 15 exceeds a predetermined level, the oscillation stop transistor 11 is turned on, whereby the base of the oscillation transistor 42 is short-circuited. Thus, the voltage boost-up operation of the DC--DC converter 4 is stopped.
When the oscillation of the DC--DC converter 4 is stopped with the ignition capacitor 5 being sufficiently charged, the ignition timing control circuit 10 outputs a trigger signal to the thyristor 8 in response to an output signal of the electromagnetic pick-up device 12 to thereby turn on the thyristor 8, as a result of which the short-circuit formed by the primary winding 7a of the ignition coil 7, the ignition capacitor 5 and the thyristor 8 is closed to allow a current to be discharged from the ignition capacitor 5. Owing to this discharge current, a high voltage is induced in the ignition secondary winding 7b, which results in generation of a spark in the spark plug 6. Parenthetically, oscillation of the DC--DC converter 4 can also be stopped by transmission of the trigger signal for the thyristor 8 to the base of the oscillation stop transistor 11 via the resistor 16.
In the conventional ignition system for the engine described above, the electric power supplied to the ignition timing control circuit 10 upon starting of the engine is delivered from the power supply stabilizer circuit 9. In this regard, it is however noted that the operation voltage of the ignition timing control circuit 10 is usually high. By way of example, in the case of the ignition timing control circuit implemented as based on a microcomputer, an operation voltage of about 5 volts is required. On the other hand, the voltage generated by the kick starter via the rectifier circuit 2 lies within a range of 2 to 3 volts, which is obviously too low to operate the ignition timing control circuit 10 without fail. As a result of this, there may arise such situation in which the engine can not be started even when the ignition capacitor 5 is charged to a sufficiently high level, because the thyristor 8 is not turned on by the output of a ignition timing control circuit 10.
As an approach to solve the problem mentioned above, there is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 124262/1991 (JP-A-H3-124262) such an ignition system for an internal combustion engine in which the electric power for the ignition timing control circuit is derived from the charging secondary winding 41b. Further, Japanese Utility Model Publication No. 21012/1992 discloses, an ignition system in which a winding for supplying electric power to the ignition timing control circuit 10 is provided at the secondary side of the transformer 41.
However, the ignition system disclosed in the first mentioned publication suffers a difficulty that the charge voltage of the ignition capacitor 5 becomes low, giving rise to a problem that spark generation by the spark plug 6 can not be ensured because the electric power for the ignition timing control circuit 10 is tapped from the charging secondary winding 41b of the transformer 41.
On the other hand, in the engine ignition system disclosed in the Utility Model Publication mentioned above, where the power supply winding for the ignition timing control circuit is provided at the secondary side of the transformer 41, there arises a problem that not only an expensive transformer of a large size is required but also a circuit configuration of the ignition system becomes complicated, incurring a high manufacturing cost.